1. Field of the Invention
The present invention relates to an image capturing apparatus, a control method thereof, and a recording medium and, particularly, to a technique of shooting binocular stereopsis images using one imaging optical system.
2. Description of the Related Art
Recently, image capturing apparatuses called 3D cameras capable of shooting binocular stereopsis images are commercially available, even for home use. As a method of shooting binocular stereopsis images, various methods have been proposed. The home 3D camera often employs a system in which two imaging optical systems are arranged at a predetermined base-line length, and left- and right-eye images are shot using the respective imaging optical systems.
As another system, binocular stereopsis images are shot using only one imaging optical system (monocular). For example, Japanese Patent Laid-Open No. 2010-81580 discloses an image capturing apparatus which acquires binocular stereopsis images by splitting a light beam which has passed through an imaging optical system into left- and right-eye light beams using a spectral mirror, and forming images from the respective light beams on separate image sensors.
In most cases, 3D cameras capable of shooting binocular stereopsis images can shoot not only binocular stereopsis images (stereo images) but also normal images (mono images).
Generally when shooting a mono image, photometry in a shooting environment is performed before shooting for recording to determine an exposure suited to photometry in the shooting environment. The shutter speed and aperture value are determined based on the exposure, and then shooting for recording is executed. For example, in shooting in a shooting environment where the brightness is high because of fine weather or the like, a sufficient light quantity can be ensured. The suppression of the light quantity of an optical image to be formed on the image sensor is therefore controlled by increasing the shutter speed (shortening the exposure time) or increasing the aperture value. Under this exposure control, an image can be shot at the correct exposure while preventing loss in highlight detail of the object.
Even when shooting stereo images, exposure control is considered to be performed similarly to the shooting of mono images. However, when shooting stereo images with a monocular 3D camera, the exposure control may cause the following problem.
When shooting binocular stereopsis images using one imaging optical system as in the image capturing apparatus disclosed in Japanese Patent Laid-Open No. 2010-81580, left- and right-eye image sensors receive light beams which have been split by the spectral mirror and which have passed through different regions of the incident pupil of the imaging optical system. For example, as shown in FIG. 7A, the spectral mirror is configured so that light beams which have passed through regions divided by a perpendicular passing through the center of the incident pupil reach the respective image sensors. In this case, optical images formed on the respective light-receiving elements are images which have passed through rectangular regions 701 and 702, as shown in FIG. 7B. At this time, the centers of gravity of the rectangular regions serve as the optical axes of the left- and right-eye images. Thus, the left- and right-eye images are equivalent to left- and right-eye images acquired by an image capturing apparatus including two imaging optical systems which are arranged using the distance between the centers of gravity (radius r of the incident pupil) as a base-line length.
Note that the incident pupil of the imaging optical system changes depending on the aperture opening. As the aperture decreases by exposure control, the distance between the centers of gravity of obtained left- and right-eye images also decreases. When the distance between the centers of gravity, that is, the base-line length decreases, a stereoscopic effect given to the observer when the left- and right-eye images are displayed on a display device capable of binocular stereopsis becomes weak.
For example, FIG. 8B shows left- and right-eye images obtained by shooting objects A and B (distance between object A and the image capturing apparatus>distance between object B and the image capturing apparatus) when the base-line length is r, as shown in FIG. 8A. At this time, if neither object A nor B exists on a plane on which the gaze points of two imaging optical systems exist, disparities corresponding to shift amounts between the images are generated as shown in FIG. 8B. FIG. 8D shows left- and right-eye images obtained by shooting objects A and B when r is r′, which is smaller than the base-line length, as shown in FIG. 8C. At this time, the base-line length is shorter than that in FIG. 8A, and disparities generated for the respective objects become small.
A human perceives the degrees of pop out or depth perception in stereoscopy depending on the magnitude of a disparity serving as a shift between images formed on the left- and right-eye retinas. As the disparity decreases, the stereoscopic effect of binocular stereopsis images weakens. That is, if exposure control is performed in the same way as that for mono images, the stereoscopic effect of stereo images on the observer may weaken depending on the aperture.